Maturation of a neurotransmitter system may not be an "all or none" event; early maturing neurotransmitter systems or subsystems could serve to mediate age-specific behaviors of the neonate independently of their function in adults. The behaviors induced by psychoactive drugs early in development often are different from those induced by the same drug in adult animals and frequently mimic environmentally-induced behaviors seen at particular stages of ontogeny. For example, potent sensory stimuli (such as novel odors, milk when deprived, foot shock) and monoamine agonists induce marked behavioral activation in rat pups during the first postnatal week, while catecholamine agonists and potent sensory stimuli induce wall climbing behavior during the second postnatal week and rarely if ever thereafter. The work outlined in this proposal will determine which neurotransmitter system(s) mediate these early age-specific behavioral responses to potent sensory stimuli and psychoactive drugs during these two specific ontogenetic periods. Different sensory stimuli and drugs will be tested for their ability to induce behaviors such as general activation and wall climbing during the first and second week of life. Then, the efficacy of different neurotransmitter antagonists in blocking both the drug- and sensory stimulus-induced behaviors will be studied to determine which neurotransmitter systems(s) are critical for the expression of these induced patterns of behavior. This psychopharmacological approach will be supplemented by analysis through brain transections, electrolytic and neurotoxin lesions, and neurochemistry, to specify which brain regions are most critical ontogenetically for these behaviors. The convergence of information gathered through these various approaches should define more clearly the role of specific neurotransmitter systems or subsystems in the behavior of the developing animal.